In the process of papermaking, it is often desirable to coat a paper sheet (called a "base sheet") with any of a wide variety of materials. Indeed, an increasing proportion of the world's paper production is devoted to coated paper and coated paperboard. Coatings are usually applied to provide a glossy white surface for magazine pages, gift wrapping, shoe boxes, and the like. Alternatively, or in addition, such coatings may also be intended to render the paper sheet waterproof. As another example of a coating material, microencapsulated ink may be applied as a coating to one side of a sheet of carbonless copy paper.
There are a large variety of coating formulations, many of which consist of as many as ten or more components. These components can be broadly classified as pigments, binders, and additives, almost always as aqueous dispersions. Common pigments include clay, calcium carbonate (CaCO.sub.3), barium sulfate, and titanium dioxide (TiO.sub.2). Barium sulfate and titanium dioxide are used only for photographic papers and specialty papers, respectively. Generally speaking, clay has been the most common pigment, although CaCO.sub.3 and PCC (precipitated calcium carbonate) are becoming more common. Various formulations of latexes are commonly used for binders to hold the pigment particles together and to bond them to the paper. A typical coating formulation includes 80% to 90% pigment, 3% to 10% latex, with the remainder consisting of additives or other components.
Such coatings as described above may be applied to paper as part of the papermaking process in a paper mill. Papermaking and coating techniques are well known in the art and are described, for example, in Pulp and Paper Manufacture, Vol. III (Papermaking & Paperboard Making), R. MacDonald, Ed., 1970, McGraw Hill and Handbook for Pulp & Paper Technologists, G. A. Smook, 2nd Ed., 1992. Angus Wilde Publications. Alternatively, previously manufactured paper may be supplied to the coating machine, called a "coater", from large rolls of paper sheet. In either event, the uncoated paper is usually supplied to the coater in sheets that are on the order of 10 feet or more in width measured along the "cross-direction" (i.e., the direction transverse to the direction of movement of the paper along the papermaking and/or coating machine).
Uniformity of coating "basis weight" (i.e., the mass of the coating material on a unit of surface area of the sheet) is often necessary or desirable for various reasons. For example, the printability of glossy paper may be improved by the uniform application of a gloss coating. Also, gloss coatings may contain relatively expensive materials, such as latex and/or TiO.sub.2. Accordingly, the manufacturer will want to precisely monitor the coating and control the application of such coating to apply as uniform a coating as possible. In some cases, the evenness of the coating must be controlled within a fraction of a gram/m.sup.2. However, because of the lateral extent of the sheet in the cross-direction (10 feet or more) and the requirement of accurately and evenly applying a coating to such sheets, rather complex coaters have been designed and manufactured.
Coaters come in a variety of configurations. One type of coater, called a "blade coater", comprises a rotating backing drum disposed adjacent to one side of a moving paper sheet and a flexible blade disposed adjacent to the opposite side of the sheet. The drum and blade edge extend in the cross-direction of the sheet to form a narrow slot through which the sheet of paper passes. A pool of coating material is retained between the backing drum and the blade, and thus coats the sheet as it passes therebetween. The blade presses against the paper with the coating applied as the sheet exits through the slot, thereby removing excess coating.
It will be appreciated that the separation of the drum from the blade edge is a critical factor in the application of such coatings. The drum is fabricated and installed to high tolerances. To control the thickness of the coating applied to a sheet, coaters provide actuators for adjusting the pressure of the blade edge against the coated sheet, and/or the position of the blade edge relative to the drum. The blade is usually made of a thin steel member which may be slightly bent or flexed. Thus, actuators are installed at intervals alone the length of the blade, such that each actuator controls the pressure applied by the blade in the vicinity of the actuator, and therefore, the amount of coating material on the base sheet. The cross-directional length of the blade in the vicinity of each actuator is known as a "slice". Commonly assigned U.S. Pat. No. 4,732,776 discloses a coater including such coating blade actuators. This patent is incorporated herein by reference.
Local variations in blade pressure and paper thickness, and possibly other factors, if not compensated for, will tend to produce uneven coatings. Therefore, it will be appreciated from the foregoing that the ability to measure the amount of coating material on the coated sheet, and to control the pressure of the blade against the sheet at a plurality of cross-directional slice positions during the coating procedure based upon such measurements will also be important to the papermaker.
Numerous schemes have been attempted to measure and control the amount of coating applied to a sheet. One of the most difficult aspects of the coating control process is obtaining an accurate measurement of the amount of coating applied to a sheet, particularly when the coating amounts must be measured to an accuracy of fractions of a gram/m.sup.2.
In one such scheme, a sheet basis weight sensor and a sheet moisture sensor are disposed upstream in the papermaking process before the coater. The basis weight sensor measures the total amount of material in the sheet in terms of mass per unit surface area. Thus, the measured basis weight includes both paper fibers and moisture absorbed by the fibers. Known basis weight sensors utilize the transmission of beta rays through the sheet to determine the basis weight of such sheet. The moisture content of the sheet may be determined, for example, by known infrared moisture sensors which similarly determine the moisture content of the sheet in terms of the mass of water in the sheet per unit surface area of the sheet. Additional basis weight and moisture sensors are then positioned at a point downstream of the coater after the coating process.
The amount of fiber forming the sheet can be determined by subtracting the amount of moisture from the basis weight of the uncoated sheet. Similarly, by subtracting the moisture content of the coated sheet from the basis weight of the coated sheet, the combined amount of coating material and paper fiber can be determined. Finally, by subtracting the amount of fiber in the uncoated sheet from the measurement of combined coating and fiber basis weight in the coated sheet, the basis weight of the coating applied to the sheet is determined. Based upon these measurements of coating basis weight at each slice across the width of the sheet, the system process control computer can then compare such measurements with a predetermined desired coating basis weight value and develop signals to control the coating blade actuators at each slice to achieve the desired coating basis weight across the entire width of the sheet.
Unfortunately, the above-described method is not completely satisfactory since it requires four relatively expensive sensors (i.e., a moisture and basis weight sensor disposed adjacent to the uncoated sheet and additional moisture and basis weight sensors disposed adjacent to the coated portion of the sheet) for determining the basis weight of the coating material. Moreover, the error inherent in the measurement of each of these four sensors may propagate additively through the mathematical calculations necessary to determine coating weight, thereby resulting in a less than ideal measurement of coating basis weight.
Another scheme for measuring the amount of coating material applied to a sheet requires the irradiation of the coated sheet with very high energy x-rays. Such high energy x-rays excite the atoms in the coated sheet material so that such atoms fluoresce. The fluorescing atoms emit x-rays having wavelengths unique to the elements in the coating. Thus, by tuning an x-ray sensor to one or more wavelengths uniquely characteristic of the elements in the coating material, the papermaker can deduce the amount of coating material by the intensity of the fluorescence at the characteristic wavelengths.
Unfortunately, the fluorescence technique is also not completely satisfactory in many instances. For example, the fluorescing atoms emit only low intensity x-rays, thus, this technique produces a relatively low signal to noise ratio. Therefore, relatively long periods of time must elapse before a statistically significant signal can be accumulated by the x-ray detector. Moreover, the high energy exciting x-rays, and the x-rays resulting from the fluorescence of the coated sheet, are dangerous to papermill personnel.
In yet another technique, portions of the sheet are irradiated with x-rays, and the intensity of the x-rays transmitted through the sheet is detected. However, x-rays are absorbed by the mineral filler material frequently used in paper sheet, the wood pulp fibers and the moisture in the sheet. Accordingly, since the transmission of x-rays through the sheet is not solely responsive to the coating material, sensors must be positioned before and after the coater, and the difference in transmission of the x-rays though the coated and uncoated portions of the sheet determined and related to the amount of coating material applied to the sheet. Again, however, this technique suffers from the deficiency that multiple relatively expensive x-ray sources and sensors are required, the error inherent in measurements made by each sensor may additively contribute to the error in the determined amount of coating, and the use of x-rays is, of course, potentially dangerous to papermill personnel.
Commonly assigned U.S. Pat. No. 4,957,770 discloses a sensor and method for determining the basis weight of a coating material by measuring radiation from the coating material. The sensor measures the latex concentration in the coating material.